My process is that I generally do all the work myself. If I'm finding that there are certain elements of the project that I can't fill or can be better filled by someone else, I might ask someone I know, or just ask around generally, if they'd like to lend a helping hand. However, this is after I've already demonstrated that the bones of the project are already in place. Even after the fact, I am still responsible for overseeing the work that other people have put in and making whatever changes are needed for quality control.
Good example:
https://mb.srb2.org/addons/chaos-mode.3010/
Chaos is crucially an online-tailored game mode, and so I had been publicizing the alpha version of it through the master server and discord so that it could get the frequent testing that it needed for the online environment it was built for. It was using Match maps at the time to spawn enemies, but I wanted the mode to have a specialized map rotation so that it could really come onto its own. I eventually advertised that I was looking for maps to add to the rotation, and I eventually got enough takers to where I had a full rotation by the end.
Three quarters of the Chaos Mode project was scripting. There are bits and pieces of resources pulled from other reusable scripts or donated by other members, but the vast majority of it is my work. The majority of the map rotation was not my work, but it was still crucial that I go into every single map and make sure that the spawns were properly balanced, since I knew my spawns better than anyone else. From this, you can see how, even in the areas where I'm ostensibly not involved, I still had to oversee and make sure everything was working right.
This is the viewpoint of a scripter and game designer, so it may vary a bit depending on whether you're a lead whose speciality lies in, say, map design or spritework. So the process I had to go through may not be necessary for everyone. But from what I have witnessed, projects seem to work out for the best when the lead understands (at least at a baseline level) the kind of modding that they are asking others to do. I am not a good map designer, but I understand how to use ZoneBuilder enough to make direct changes or make suggestions to the main author/editor for that level. You do not need to be a good scripter, but it wouldn't be a bad idea to develop a baseline understanding of how Lua works -- not enough to make your own scripts necessarily, but at least well enough that you can communicate with your scripter to solve design or technical issues, and to understand what kind of scripting is feasible in the first place.
Beyond that, I can only echo what Charyb was suggested; demonstrate that you can put in the work, and invariably someone will be interested enough to lend a hand.
